You can use console commands to affect gameplay, ban users and view various information. There are mods that expand on the vanilla Valheim commands and add new ones.
To open or close the command console, press F5. Please note that the in-game console is disabled by default as of update 0.148.6. To enable it, add launch argument "-console" to Valheim in Steam. This is done by right-clicking Valheim in the Steam window, then going to properties, and then pasting "-console" (without quotations) in the "Launch Options" field.
help - Shows all the available server commands
info - Prints the current system information.
ping - Pings the server to measure latency.
Admin Commands (How to make yourself admin)
ban [name/ip/userID] - Bans the named user.
banned - Shows a full list of banned users.
kick [name/ip/userID] - Kicks the named user.
lodbias [number] - Sets the draw distance for the server. The number can be set from 1 to 5.
save - Forces the game to save.
unban [name/ip/userID] - Unbans the named user.