Replace the rubber washer inside the hose’s female end. The washer makes the seal, so a missing or worn one leaks however hard you tighten.
Check these in order
The washer is missing, flattened or cracked
How to tell: Turn the faucet off, unscrew the hose and look inside the female end. You should see a flat rubber ring sitting flush. Missing, hard, cracked or squashed means replace it.
Fix: Pry the old one out with a flat screwdriver and press a new one in. Screw the hose back on by hand until the threads catch evenly, then snug it about a quarter turn past hand-tight. If the fitting seals only with serious force, check the washer and threads again.
The threads are bent or worn
How to tell: Look at both sets of threads. Flattened, cracked or chewed threads on a plastic or thin metal coupling are the second most common leak, often from tightening with pliers.
Fix: Replace the damaged coupling. A hose mender or coupling repair kit fits the hose end; a damaged faucet spout needs the faucet repaired or replaced.
It went on crooked (cross-threaded)
How to tell: The hose stopped turning early, or sits at a slight angle on the spout.
Fix: Unscrew it, line it up straight and start it by hand. It should turn freely for several turns before it gets tight.
A hose thread is joined to a pipe thread
How to tell: Something in the chain (a timer, splitter or adapter) has pipe thread, labelled NPT, MIP or FIP, instead of hose thread.
Fix: Use a proper hose-to-pipe adapter. Hose and pipe threads can’t be interchanged, and Teflon tape won’t fix the mix. See hose thread vs pipe thread.
Water coming out of the top of the faucet
Many outdoor faucets have a vacuum breaker: a cap on top that stops garden water flowing back into the house. Water leaking from the vacuum breaker while the faucet is on means it is fouled or damaged. The fix is to clean it or fit the maker’s vacuum breaker kit. Watts hose vacuum breakers are not for use under continuous pressure.
Hose leaking where two hoses join
The same first check applies: the female end needs a good washer. Garden hose thread is a straight thread. Hoses made to the British BSP standard “may interconnect in a rough manner” with a US hose, relying on the washer to stop leaks. See connecting two garden hoses.
Thread tape won’t fix a hose leak. Start with the washer.