"Top ups" refers to the act of adding a small amount of something, usually money, to an existing amount. It is like filling something up partially or making a small addition to it. Full definition
He can make the additional investments by paying unlimited top up premium amounts to increase the fund value, given that all the claims have been done so far. (policybazaar.com)
Top up for Future Income and Guaranteed Income Advantage premiums, is an extra amount of money that you can pay at any time during the policy term. (life.indiainsured.com)
T - Mobile offering in the pay as you go market comes with 6 months free internet and BlackBerry mail, with unlimited texts if topped up with # 10 a month. (onemobilering.com)