From what I can patch together,
the FBI was investigating former congressman Anthony Weiner for potential crimes involving sexting with an underage girl. As part of the investigation, the FBI seized Weiner’s laptop to search it for evidence of the sexting crimes. I would guess, although I haven’t yet been able to confirm, that the FBI obtained a warrant to search Weiner’s computer. The Fourth Amendment would generally require a warrant to search a suspect’s personal computer unless there are special circumstances such as consent that haven’t been mentioned in press reports.
The case connects to Clinton because the laptop happens to have been shared by Weiner and his now-estranged wife, Huma Abedin, who is an important adviser to Hillary Clinton. In the course of searching Weiner’s laptop, the FBI came across emails in Abedin’s email account that appeared to the agents to be relevant to the Clinton email server case.
According to news reports, the FBI now is planning to get a warrant to search the laptop for emails related to the Clinton server case. They haven’t obtained that warrant yet, however, so the Weiner computer has not yet been subject to a comprehensive search.