bathtub drain leaking
Old plumber's putty will dry out and the tub "flexing" due to people and water weight in the tub can cause it to start leaking.
To seal the tub drain, you just need to unscrew the drain counter-clockwise, clean out the old plumber's putty and replace it.
If you don't have the proper drain tool (most don't), you can use the handles of a pair of channel-lock pliers down in the crosspieces of the drain and use a screwdriver in the plier teeth for leverage to unscrew the drain.
You might want to loosen the old putty by heating it with a hair dryer first.
If you break out the crosspieces, you will have to buy a new drain.
Once you've cleaned off all of the old putty, roll a roll of new plumber's putty about the size of a pencil in your hands and wrap it around the depression around the tub drain hole, then tighten the drain down on it.
We need to know the brand and type of faucet before we can help with that. Single handle? Dual handles?