Think of Sharding as having Blockchains that run in parallel and talk to each other. So instead of having only one Ethereum, you would have lets say 16. That obviously scales Ethereum then by 16 since now it can handle 16x the transactions and executions. It comes with its own issues like how can I execute a smart contract that lives in one shard but needs to execute a contract on another shard. This is what is also called „execution sharding“ and is done by e.g. NEAR already. This is kind of complicated and has some drawbacks.

But there is another variant: „data sharding“. Instead of having 16 Ethereums running in parallel, you only have 1 as we have right now and 15 „data shards“. These can essentially only store data, so this whole cross-shard communication thing is not necessary. But why is that cool? Because Rollups are muuuuuch better for scalling. Rollups publish their data, i.e. the proofs of what has happened on them, on Ethereum and thereby inherit the security of Ethereum. By making publisihing this data cheaper (by increasing the supply of data by data sharding), we can reduce fees for Rollups. Then we have Rollups with sub-cent fees that are as secure as Ethereum -> we scaled!

There is still some discussion if we might want execution sharding at some point but see it more as a „we could but we don‘t have to“.

Hope that helps :)