How do I change custom access to admin access and what does custom access entitle me to do

I have a Sandisk 5TB G series external hard drive that was originally formatted for Mac but I've always used it on my PC after changing it to ExFAT. which can be utilised on both Windows and Mac. I am now heading overseas and want to use the external hard drive for extra storage with my Macbook Air 3 - 512GB. It seems I only have custom access. What does that mean? How can I get admin access? Bearing in mind, I have only just bought a Mac laptop again after buying the last in 2013. I've never been familiar with how permissions and access work on a Mac. If somebody could give me some advice about where to go to check the hard drive's status, seeing what permissions custom access gives me and how I could change that to admin - I am the owner of the HDD and only user and will only ever be the only user.