The 1933 Act is about the primary market. Anyone selling a security to the public must register the offering, which in practice means filing a form-s-1 and delivering a prospectus, or fit within an exemption such as a private placement under private-placement-regulation-d.
Its liability provisions are unusually strict. Section 11 lets purchasers sue over material misstatements in a registration statement without proving intent or reliance, and directors, signing officers, underwriters and named experts are all on the hook unless they establish due diligence.
Resale restrictions follow from the same logic. Securities sold under an exemption are restricted and can generally only be resold under rule-144 or another exemption, which is why insiders and private placement investors cannot simply sell into the market.
Related: securities-exchange-act-1934, form-s-1, prospectus, rule-144, private-placement-regulation-d