Käydä is an old fashioned word for “to walk”. More common way to say “to walk” is “kävellä” (which is of course derived from the earlier verb). Käydä is still in use in some special meanings, where the idea is more like “to visit a place”. For example: käydä kaupassa / do the shopping, käydä lääkärissä / to see the doctor (in Finnish it’s just enough to see him).
In English “engine runs”, but in Finnish it “walks”, so moottori käy tasaisesti / engine runs smoothly. Also beer and wine “walk” when it ferments, so “hyvä viini käy vuosia / good wine ferments for years”.
If you want to say, that something is ok, you can say “it walks”, “se käy“. Other possibility is to say “se sopii“, “it fits”. Both are standard Finnish, not slang.