It sounds like you have standard adjustable pickups. These might feel loose when you push on them, but shouldn't "flop" around by themselves. An adjustable pickup usually has a spring between the pickup and pickguard so that you can adjust the height and tilt. The adjusting screw runs through the spring such that the pickups are "floating". The height is adjusted for the volume and sustain that you desire, while the tilt is adjusted for balance between strings. You might also be able to adjust the individual pole-pieces for balance between strings.

If you adjust the pickup closer to the strings, you will increase the volume. However, if the pickup is too close, the magnet will dampen string vibration and you will lose sustain. If the pickups are too far away from the strings, you will have low volume and the pickups may move around if the spring is completely uncompressed.

My Strat has fixed magnetic pole pieces so the two screws on the side of the pickup are used to adjust pickup height and tilt. My Les Paul has adjustable poles, so the screws are used to adjust height while the poles are used to balance the signal between the strings.