This would be an end game upgrade that would allow you to store up to 2 charges of the portal. For the sake of argument, the portal currently has a 1h cooldown.
Once the upgrade is purchased, it immediately starts a 1h cooldown. If the user does not use a portal before the 1h is up, the portal now has two charges.
When the portal is used, 1 of the charges is consumed and it immediately starts recharging. When the second charge is consumed, you'd then see the count down timer of when the first portal charge comes back.
It's honestly not really necessary, but I thought it'd be a neat QoL change for players that are at the end, end game.