The exact language that is accused of being anti-Semitic is:
"They [Senators pushing a bill to deter US companies from boycotting Israel] forgot what country they represent. This is the U.S. where boycotting is a right & part of our historical fight for freedom & equality. Maybe a refresher on our U.S. Constitution is in order, then get back to opening up our government instead of taking our rights away."
This tweet was in response to a tweet by Bernie Sanders about a bipartisan bill in the Senate to attempt to make boycotting Israel more difficult. The particular aspect of this that is challenged as anti-Semitic is "they forgot what country they represent", which can be seen at playing in to dog-whistles and stereotypes of Jewish people having two loyalties.
Whether this is antisemitic or not is a difficult question, compounded by most of these senators supporting the bill not being Jewish (that is her line, that it was about the Senators, not Jewish people). In general, distaste for the Israeli government and antisemitism are somewhat difficult to disentangle, and the matter is especially sensitive for someone of Palestinian descent such as Representative Tlaib. Her critics see it as anti-Semitism, her supporters see it as standing up for human rights.
Note that an insinuation or implication can be a slur, although it may not be as obvious as pejorative language.