Fanservice basically just means catering to fans in ways that are pleasing to them. It can be as innocent as a simple handshake, photograph, or autograph session with fans. There are also more sexual forms of fanservice which can draw negative attention from non-fans.
Have you heard of the term “shipping”? The word “ship” originates from “relationship”. Fans enjoy pairing up celebrities they like and desiring them to be in a relationship with one another is what shipping means. Some more hardworking fans would concoct and write fanfiction stories regarding their favorite ‘ship’. Fans shipping their favorite idol of the same gender are common from fans of the opposite gender. This is presumably because the celebrities would still have “relationship availability status” to fans of the opposite sex.
Take for example AKB48, an all-girl band whose contracts do not allow them to date guys or have boyfriends. Thus, they take advantage of the relationships between girls to entice their fans such as in the advertisement pictured above for Puccho candy.
In the advertisement, the girls passed candy to each other using their mouths. The ad sparked controversy in Japan was criticized as a bad influence for children due to the unhygienic act which was also considered to be “encouraging homosexuality.”
Even male idols and artists do similar things to drive fangirls wild. Here are photos of Gackt kissing and hugging his guitarist at his concerts:
Another common method by idols to provide fanservice is to release photobooks. The photobooks of female idols tend to be filled with sexy images like bikini shots and very little else.
The response from fans may be positive, but some non-fans may not take to the fanservice as kindly. While it is mostly used as a tool to boost popularity, there are some who find it too attention-seeking. The average non-fan might be unwilling to pay attention to these idols as a result.