Question about common lewis structures film from the course

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Hi, in the course in the film about imat 2020 Q44 i found sentences:

  1. Carbon dioxide (CO2) is a common molecule used to confuse students because it has a linear shape, and the vectors inside the molecule cancel each other, making it nonpolar.
  2. Methane (CH4) has a tetrahedral shape, and its vectors cancel each other as well.
    which i do not understand- what cancel each other mean here?
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The electron cloud around the central C-atom is symmetrical in both cases. This is why the overall polarity of the molecule is neutral.

Molecules like HF or CH3OH are polar because of asymmetrical electron could which is more intense around the more electronegative atom inside their molecule, in this case F and O respectively.

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