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Selenium Grid is a server that allows tests to use web browser instances running on remote machines.
With Selenium Grid, one server acts as the hub. Tests contact the hub to obtain access to browser instances. The hub has a list of servers that provide access to browser instances (WebDriver nodes), and lets tests use these instances.
Selenium Grid allows running tests in parallel on multiple machines, and to manage different browser versions and browser configurations centrally (instead of in each individual test).
The ability to run tests on remote browser instances is useful to spread the load of testing across several machines, and to run tests in browsers running on different platforms or operating systems. The latter is particularly useful in cases where not all browsers to be used for testing can run on the same platform.
Whether you should know about Selenium Grid is dependent on your current task.
Read it more here. Link to Selenium Grid Documentation