Yes it's a tyre wear indicator and just FYI, MOT rules and regs state that there has to be legal tread across 75% of the tyre surface. You can have an almost bold inner/outer edge and still be legal. Doesn't mean you'd wanna anchor up hard or try some enthusiastic driving on them.

There near legal limit, get them changed.

£2500 fine and 3 points per tyre if the plod catch you