Could anyone help me explainunderstand the meanings of themse sentences?

You're understanding, we're explaining. So, you have to say either "Could anyone explain" or "Could anyone help me understand". You can't say "Could anyone help me explain" because you're not the one explaining.

It was a rainy day, and the coffee shop was really slow, so the day really dragged for the workers.

"the coffee shop was really slow" means things (just events) were going really slow that day at the coffee shop. "the day dragged (on)" means that the day felt longer for the workers.

It cost an arm and a leg to fix the stove.

"an arm and a leg" are a lot to sacrifice, so it's used as a metaphor/idiom to mean "a lot (of money)". "fix the stove" literally just means to "fix" (repair) the stove.