That tweet is probably referring to this complaint filed on 9th of November which is effectively calling into question the fairness and legality of some practices of some counties in the Pennsylvania election.
The portion of the complaint he's probably referring to is the allegation that 682,479 mail-in ballots were illegal.
Rather than engaging in an open and transparent process to give credibility to
Pennsylvania’s brand-new voting system, the processes were hidden during the
receipt, review, opening, and tabulation of those 682,479 votes in direct
contravention of the Election Code.
I feel these quotes summarise the part relevant to the tweet:
The right to vote includes not just the right to cast a ballot, but also the
right to have it fairly counted if it is legally cast. The right to vote is infringed if a vote is cancelled or diluted by a fraudulent or illegal vote, including without limitation when a single person votes multiple times.
treatment of Pennsylvania voters, in subjecting one class of voters to greater burdens or scrutiny than another, violates Equal Protection guarantees because “the right of
suffrage can be denied by a debasement or dilution of the weight of a citizen’s vote
just as effectively as by wholly prohibiting the free exercise of the franchise.”
Through the arbitrary and illegal actions of the Secretary, Pennsylvania
created a two-track system of voting resulting in voters being treated differently
depending on how they chose to exercise their franchise. The first, marked by voters
appearing personally at the polls complied with transparency and verifiability
requirements of Pennsylvania Election Code. The second, marked by a mass of paper
ballots received through the mail, was cloaked in darkness and complied with none
of those transparency and verifiability requirements. This two-track election system
not only violates Plaintiffs’ rights guaranteed by the United States Constitution, but
also violates the structure of the Constitution that elections in the States must be
carried out as directed by their respective legislatures
There is a lot more contained within the complaint filed, but I don't think it's directly relevant to this the tweet in question, as there's also a broader complaint that mail-in voting was illegitimate contained within the same complaint. If you'd like more context I'd recommend reading through the introduction part (up to page 8) as it's quite easy to understand.
Remember that until this has gone through the courts nothing is official - it's an important part of the election process that an incumbent President cannot directly interfere. The only thing you could really read from his tweet with some degree of confidence is that Trump is feeling pretty positive about his chances.