Each month, you will have a set amount of tokens available that can be claimed according to the presale-specific vesting schedule. If you forget or otherwise choose not to claim your available tokens in a given month-long period, you will NOT be able to claim those unclaimed tokens until the last month.
To give some examples:
Let's say you forget to claim your tokens for a given month-long claim period. You will NOT be able to claim two months' worth of tokens in that subsequent month, rather, those tokens you didn't claim will be added on to the LAST claim period in your vesting schedule.
Let's say you forget to claim your tokens for three months and try to claim four months' worth of tokens on that fourth month. You will not be able to do so, as those first three months' claims will be added on to the last claim period. You would only be able to claim that month's tokens at that time.