Another player in the hand made an 'under raise'. An all-in bet of less than the size of the minimum allowed raise is considered an 'under raise' and for betting purposes is treated as a call. This means that it does not re-open the betting action for anybody that has already acted on their hand in that betting round, and as such they can only call or fold and cannot raise.
The bet slider ranges from the minimum a player can bet to the maximum a player can bet. The default bet is half the size of the pot. The default raise is also half the size of the pot.
The marker on the bet slider represents the size of the current pot.
There are also buttons for pot (to bet or raise the size of the pot) and Max (to bet the maximum allowed).