If you've got a storage type water heater they have a sacrificial anode inside to reduce corrosion of the actual heater tank. They're meant to be replaced at intervals (around five years typically) to make the heater last longer.
I changed mine today and it was showing very noticeable wear, I doubt anyone had changed it since the heater was put in around ten years ago (in part because it's installed in an awkward position under the sink so you have to remove the sink to change the anode).
Comparison photos between the old and new anodes:
Technically you should probably get a plumber to do this but it's easily done if you've got a decent sized socket set and are cluey enough to realise you should turn the inlet tap off and have a pull at the pressure release valve before you open the tank.
I replaced mine this spring. Once I got the cap loose from the rust on the threads, there was.... absolutely nothing left of the anode. A nub just below the cap maybe.
For the diy-ers, you didn't mention draining the system before you open the anode cap on the heater.. Turn off the intake water at your heater, turn down or off your heater, go to the furthest upstairs/away from your water heater faucet and turn on the hot fill blast, and back down to your water heater and open up the drain spigot (ideally with a hose straight to a drain or out to the gutter), and let that run out for a while. This way you won't get the entire house-pipe's full of hot water coming out of you anode hole. Ask me how I know 💦
While you're doing water heater maintenance, don't forget to purge the heater after you're done replacing the anode. (Turn the heat way down or off, turn off the water intake, release the pressure if you haven't already, drain out like 5 gallons from the heater, turn on the intake for a minute, turn it off, drain, on, a few times until the water coming out the heater is clear.) It's worth the time to get the sediment and rust and whatever out.