If you hold some assets in the wallet, you might have noticed that some of your ADA is locked. The reason behind this is that storing any assets on the Cardano blockchain "costs" some money.
You are not actually paying it to anyone, instead, you are just depositing them as collateral to ensure that you have some motivation to not leave lots of worthless assets just lying around forever and forget about them. It also protects the Cardano blockchain from growing intractably.
At the moment it takes at least 1 ADA to store any ADA, which might sound weird, but in reality, it just means that you cannot send any transaction of less than 1 ADA (for now, but it can be changed at any moment as a simple protocol parameter).
Similar to that, storing any native tokens or NFTs requires having a minimum deposit of ADA. The number of ADA required to store them is called the minimum ADA value and it depends on the byte size of the native tokens. Roughly, it's around 1.3-1.5 ADA.
Finally, the Locked Assets Deposit is the number of the ADA you need to store all of your assets in the wallet (you cannot use it for ADA transfers, so sometimes you may see an error message that you don't have enough funds for a transaction even though you think there's enough ADA) and it depends on how many assets you have and how closely the assets are stored in a wallet (due to the UTxO model of Cardano).
For example, you may have a UTxO entry with one NFT that requires 1.5 ADA to store it, but storing 5 NFTs together in one UTxO might require ~3 ADA in total, instead of 7.5 ADA in 5 separate boxes. For more details about the UTxO model and why the locked deposit number may change, please follow this link.