One of the most historic slogans of the movement related to Bitcoin is "Not Your Keys, Not Your Coins", so, if you do not have full possession of the keys (passwords and seed phrase) of your wallet, you do not really own your cryptocurrencies.
In other words, if you store your cryptocurrencies on a centralized excange-or any third-party custodial service, you essentially do not own them; it is the same principle as bank accounts: the money in the bank is not yours, but at the same time you have a claim on the bank for the equivalent of your deposits.
The inherent security of blockchains is not exactly the same security of centralized exchanges, so the less valid and careful ones have been "looted" over and over again. Therefore, personal online or offline wallet solutions, flash drives and various services have spread.
In this framework, the Metamask wallet is a Chrome browser extension that allows for autonomous operations on a number of crypto networks starting with Ethereum and is essentially the gateway to enter metaverses and gaming environments or apps, that do not require data, but identify the person based on their wallet and will regulate all operations for those who demonstrate they have the wallet keys.
Metamask has a very important role in DEFI in that it allows to exchange tokens within the app, connecting with specific DEFI pools or services but for example it does not connect with some specific chains, among the best known Solana, and it does not allow you to view your NFTs directly. However, it is of immediate and widespread use (synonymous with security).
Metamask is also an EVM (Ethereum Virtual Machine) wallet , so in order to use a network, it must be compatible with Ethereum, or rather EVM, using Solidity as language.