From the Account Management - Apple Developer support page:
Resetting Your Device List Annually
If you are the Account Holder, you can reset your list of development devices each year using Certificates, Identifiers & Profiles. You can register up to 100 of each device type for testing and Ad Hoc distribution per membership year. At the start of your new membership year, Account Holders, Admins, and App Managers will be presented with the option to remove listed devices and restore the available device count to 100 when first signing in to Certificates, Identifiers & Profiles.
Although you may remove a device from your account during the year, it will continue to count against your device limit. When resetting your list, make sure to remove all devices you no longer use for development before adding any new devices.
Now, to answer your question:
I am just not sure what this reset button will do. Delete all the devices or delete selected devices?
You can either remove all or selected devices.
Once every year, at the time of renewal of your Apple development program membership, you are given an option to remove registered device UDIDs and add new ones.
After you click on the Reset button, you'd be presented with the list of all the devices currently added in the Developer Portal. You can choose what devices to carry over in the next membership year and what devices to drop. You can claim the slots occupied by the devices that you no longer wish to use in future.
During the membership year, removing devices doesn't automatically free up the slot occupied by them (you get a limit of 100 development devices per year).
So if you have reached the limit and want to replace some of the devices with newer/different ones, this is your once in a year chance to do so.
I cannot risk clicking on the button without knowing what it does since it's a company account. Can anyone please help me with this?
Clicking the reset button won't immediately reset the list of devices. So you need not worry about it. You'd be resented with on-screen-instructions to help with further action.
It would be advisable here to select and remove only those devices which are no longer being used for app development and/or testing. This would free up an equal number of slots for adding new/other devices during the year.
Note: Removing a device does not prevent you from re-adding it into the Developer portal.