à fond = très vite, très rapidement
Il roule à fond / il conduit à fond = he's driving very fast
Sometimes, "à fond" also means "fully", like if you say "je profite à fond", it means "I enjoy fully".
"Il met la musique à fond" = "he plays musique very loud".
"- tu aimes le tennis ?
- à fond !"
- do you like tennis ?
- yes, so much !
So, in a business context, you'd ask someone how is the business going.
The person would answer "ça marche à fond !" like, "it works fully", it goes very well, it goes perfect.
Is that clear ?
If no, LMK and I will tell you more :)