Meaning of the parameter in a line-plane intersection in the space
The $t$ parameter represents the distance from the ray's origin point to the intersection point, as a multiple of the ray's direction vector. It is positive for intersections "in front" of the origin, negative for those "behind" the origin as judged by the direction vector.

In raytracing we always discard any intersections with $t &lt; 0$, because we don't want to render objects behind the camera, and so forth. A ray is only half a line, and only the points $t \geq 0$ are part of the ray.

It is not correct to say "if $t&lt;0$ the point will stay in a part of plane that does not contain the polygon". If the polygon is behind the ray origin, then the intersection point may well be inside the polygon. However, we can immediately discard this intersection due to its being located behind the ray origin, so we would not need to perform any further tests of whether the point on the plane is in fact inside the polygon.