we have a problem with our internet connection. it is not stable and not as fast as it can be... Our provider suggested we contact an electrician to " break" (?) The bridge tap. According to him, it can help. Before I contact an electrician, I wish to understand a little bit about that and to be sure it makes sense? Thank you for your help!
The bridge tap basically just means that you have your DSL modem connected to a pair of phone wires that also branches off to (probably) other jacks in your home. All of those jacks are "bridged" together. This could literally be crimped or twisted connections inside your walls, or you might have a wiring closet with more structure, such as all the wires screwed down to posts on a bus. This is presuming that you're not on a party line and don't have law enforcement or a nosy neighbor physically tapping your phone line (humor?).
Have you recently plugged anything else into any other phone jacks in your home? If you have, then unplug it and see if your performance improves.
Even without other devices plugged into the extension jacks, a "bridge tap" can cause reflections echoing around on the wires which can interfere with the performance of your DSL modem.
What they're really telling you to do is to have a single, unbranched, dedicated phone line for your DSL modem (2 twisted wires technically, but I'd just think of it as a single thin phone cable).
An electrician or knowledgeable friend could help you with this. This is low-voltage, so the installer doesn't really have to be a licensed electrician unless your locality requires it.
Since you said the connection was fine and now suddenly is not, I don't think their suggestion is worthwhile. Even if it was it seems more of a phone company issue rather than electrician but maybe that's different in Switzerland. See https://en.wikipedia.org/wiki/Bridge_tap
I would suspect either your modem (and/or router if a separate component) has developed an issue (I've had several over the years that have degraded over time - possibly due to heat) or a phone line issue (I've had to have our line repaired at least twice in the 20 years we've been here). Could possibly be the filter used on the phone line but I've never heard of one of those going bad.
If you have not already done it, power off your modem and router. Wait 30 seconds and power on the modem. Once it has fully connected, power on the router. If that doesn't fix anything, see if you can borrow a friend's to test.