You'd have to ask experts on contracts and cap space like
@Sarni and
@Eboue but I think it pretty much means they're locked in for the next few years and are in win (or at least compete) now mode.
I don't mind the Simmons and Harris deals, but thought that giving a 4-year, $100m+ deal to a 33-year old Horford limited them a bit towards the future. I think all of Harris, Horford, Embiid and Simmons are now locked until at least the 22-23 season, so that's a 4 season window they have with their core group now. They also have Josh Richardson at an extremely kind contract, something like a 2-year, $10m deal. Think there was a player option after those 2 years but he'll have lots of opportunities to cash in big if he keeps playing like he has been. That's a worry for in two years time though for the Sixers.
If they felt that Simmons is their franchise PG going forward, it's not a bad contract. I have my reservations about him but if they didn't sign him he'd have gotten the contract elsewhere no doubt.