What does it mean for a patent application to be ‘allowed’ or ‘in issue’?

According to MPEP 203.04:

An ‘allowed’ nonprovisional application or an application ‘in issue’ is one which has been examined and determined to meet all statutory requirements, and in which a notice of allowance has been sent to the applicant. Its status as an ‘allowed’ application continues from the date of the notice of allowance until it issues as a patent, unless it is withdrawn from issue (37 CFR 1.313), or becomes abandoned for failure to pay the issue fee and any required publication fee (37 CFR 1.314 and 1.316).

An ‘allowed’ application has passed examination and is ready to become a patent, pending payment of necessary fees.