What does this mean?

[Comfort859]

customer paid an invoice but it is showing "pending " on their end and when I open the sent invoice it still shows nothing has been paid. 

[Minion]

I've never ran into that issue, so taking a quote from this thread here,

"An invoice will show paid immediately and will reflect in Transactions. Sounds like the invoice payment declined - could be mismatched card info entered or a generic decline from the card issuer. Check Dashboard > Reports > Transactions Status to confirm if the payment was indeed unsuccessful.

Nonetheless, an unsuccessful invoice payment may appear as pending on your customer's bank statement but will fall off soon depending on their bank's processing speeds. "

Also what type of card did they use?