I was in the same position yesterday.

I opened an effect (SampleTank 3) within BIAB and at the bottom of the new window it said that JBridge was active.

What is it?
Some programs are compiled to run with a bandwidth of 64 bits others need only 32 and even older programs only 16 or 8 bits of bandwidth. (See it as a highway with as many lanes going in the same direction.) Here we only look at 64 and 32 bits.

This bandwidth is not only that more data can be processed at the same time it also means that there are more addresses available in the memory (RAM) of the computer.

Very often now 64-bit and 32-bit programs don't like each other and don't work together. JBridge mediates between those two and helps to bridge tha gap. So when you are using a 64-bit operating system JBridge lets 32-bit programs work together with 64-bit applications and 64-bit programs with 32-bit applications.

Band in a Box is a 32-bit program. Currently it doesn't need more bandwith. E. g. SampleTank 3 on the other hand is a 64-bit application. To have BIAB in a position to use SampleTank 3 you need somebody to negotiate between those two. JBridge does that.

If you don't use a 64-bit application inside of BIAB or RealBand it does no harm to have it installed.

Desktop; i7-2600k, 8 GB mem., Win 10 Pro, BIAB 2017; RB 2017 - latest build
Laptop: i5-2410M, 4 GB mem, Win 10 Pro, BIAB 2017; RB 2017 - latest build