what is meant by the word - threshold?

Apart from the physical object we consider threshold as. I think it also means as a "purpose" or "starting point"

This is what i found on the dictionary. In general, a threshold is a fixed location or value where an abrupt change is observed. It may also have other meanings.

Technical
In architecture, a strip of wood, metal, rubber or stone located on the floor in a doorway following the line of division between the two spaces linked by the doorway.
In aviation, a threshold is the end of a runway over which an airplane first passes when landing.
In biology, an action potential is initiated if the membrane potential is depolarized to the threshold potential.
In chemistry, threshold energy is a synonym for activation energy, the minimum energy necessary for a specific chemical reaction to occur.
In cryptography, a cryptosystem is called a threshold cryptosystem, if in order to decrypt a secret a number of parties that hold a key share exceeding the threshold is required.
In elections, election threshold refers to the minimal amount of votes necessary to win representation.
In electrical engineering and machine learning, a threshold circuit or unit is a one-input, one-output device or object that implements a Heaviside step function; the threshold value corresponds to the transition in this function between one and zero.
In occupational safety and health, see ACGIH and its Threshold Limit Values (TLV).
In optics, the lasing threshold is the minimum power level at which a laser starts to operate.
In psychology, sensory threshold is one of the nine aspects of temperament. The minimum amount of stimulus energy necessary to elicit a sensory response.

Telecommunications
According to Federal Standard 1037C and from MIL-STD-188, threshold can refer to the minimum value of a signal that can be detected by the system or sensor under consideration.
a value used to denote predetermined levels, such as those pertaining to volume of message storage, i.e., in-transit storage or queue storage, used in a message switching center.
the minimum value of the parameter used to activate a device.
the minimum value a stimulus may have to create a desired effect.

Popular culture
Threshold is a science fiction novel by David R. Palmer.
Threshold is also the name of a fantasy book by Sara Douglass.
Threshold is a progressive metal band from England.
In the video game "Halo", Threshold is the planet around which the ringworld Halo orbits.
The Sci Fi Channel aired the movie Threshold in 2003
Threshold is the name of a science-fiction television series that began airing in 2005 on CBS and now airs on the Sci-Fi Channel
"Threshold" was an episode in Star Trek: Voyager.
Threshold RPG is a text-based roleplaying MUD game.
Threshold is a comic book character from Wildstorm
The Threshold is an organization of Gallifreyan/Human origin in Doctor Who comic strips in the pre-2005 Doctor Who Magazine.
Threshold is the name of last album from the band HammerFall, released in 2006.

門檻 -
He had never crossed the threshold of a bar before.
他過去從未進過酒吧的門。


出發點,開端,起點 -
Scientists are on the threshold of a breakthrough in their treatment of that disease.
科學家在治療那種疾病方面很快就要取得突破。


稅的)起徵點 -
The tax threshold for a pensioner is $6,000.
領退休金的人的納稅起點是六千美元。


工資,價格的)起增點

threshold 最起初的意思是門檻
但它卻被引申其他解釋，如在物理，或電子學科上也有用上threshold，但在這個情況下當然不是解作門檻。
因為人都必需把腳提高過門檻才能進入，所以 threshold，就被引申解作”產生某樣事情的最低條件”，如在某些電子零件，我們起碼要給它input voltage　３ｖ，它才能運作，那時這零件的 threshold 就是３ｖ。希望這樣解釋你會明白。

It means 門檻. It also refers to the barriers of entry a industry.

詞-門限是什麼意思？
門限能是您看見或記得的事當您步行沿著向下街道時？

Threshold is the bar (made of either stone or wood) forming the bottom of a doorway or entrance, you should see them at the entrance of any shops in the street

I don't think its a physical thing that you actually see. Cos we usually use this word to describe some sort of barrier/starting point that you need to oversome.