A bridge on a string instrument is something that stands on the body of the instrument and supports the strings. It sends the vibrations of the strings to the main part of the instrument so that the sound can be heard.
The bridge of an instrument in the violin family (violin, viola, cello and double bass) can be seen very clearly. It is a piece of hardwood which is shaped like a bridge. The top is like an arch in shape because the player needs to be able to play on one string at a time. The feet of the bridge have to be shaped to fit on the front of the instrument (the “belly”) which is curved. The bridge is not fixed to the instrument: it is only held up because of the tension (tightness) of the strings. The soundpost, a small wooden column inside the body, is also held up by tension of the strings and the bridge. The part of the string which vibrates and gives the note is the part between the bridge and the top of the fingerboard (or the part between the bridge and where the player puts his finger). The other end of the string is fixed to a tailpiece.
On a banjo the bridge works in the same way, but on a guitar the bridge is fixed to the instrument and the strings are fixed to the bridge.
There are even instruments with moveable bridges such as the Japanese koto which has a separate bridge for each string.