Parliament was elected in 2015. By 2017 — of 650 seats, the Tories had 330. Not a comfortable majority. Even though Labour, the next largest party, had only 229 seats. Several parties get members elected to Parliament.
Parliament has a five year term. Elections were not scheduled until 2020. The PM can call an election before the scheduled election. Theresa May announced a new election in 2017. She wanted a larger majority That is not what she got.
Here is what she had when she resigned:
Tory (Conservative) 313
Labour (Socialist) 247
Scottish National Party (for Scottish independence) 35
Independent (no party) 15
Liberal Democrat (not socialist) 12
Democratic Union (Northern Ireland) 10
Sinn Fein (Northern Ireland, favor union with Ireland) 7
Change UK (defectors from Tory and Labour) 5
Plaid Cymru (Welsh nationalists) 4
Green Party 1
Speaker 1
Total 650
Theresa May had formed a government by making an agreement with the Democratic Union Party of Northern Ireland. That is only 323 votes, 3 short of a majority. Sinn Fein Party members refuse to join Parliament even if they are elected because they refuse to swear loyalty to the Queen. Effectively then, the total is 643, a majority of which is 322.
The Tories remain the governing party. The winner of what I described in US terms as a national primary of the Conservative Party will be known in July. Boris Johnson, the Flamboyant. A better educated version of Donald Trump. Versus Jeremy Hunt the Mild. Currently the Foreign Minister. Will he be a patsy for Johnson?