what does replicatedfirst does?

It IS important that you know what it does. ReplicatedStorage is called so because descendants of it not only exists on the server, but "replicate" to every client that joins the game. On the other hand, everything in ServerStorage exists on the server, but doesn't replicate to the client. Also note that descendants of ReplicatedStorage and ServerStorage will essentially be in a state of stasis, unlike if they were in Workspace.

EDIT: ReplicatedFirst is essentially the same thing as ReplicatedStorage, in that its descendants replicate to both the server and the client, except that it takes priority between the two, and as the name implies, its descendants are loaded before those of ReplicatedStorage, making it ideal for holding things that concern the client, such as LocalScripts.

https://developer.roblox.com/en-us/api-reference/class/ReplicatedStorage https://developer.roblox.com/en-us/api-reference/class/ReplicatedFirst