If you heard this word, it could have been in regard to either a light source (Lambertian emitter) or a reflective surface (Lambertian reflectance). In either case, Lambertian means light rays are going evenly in all directions (kind of). So no matter if your eye is at 0 degrees to the thing, or 80 degrees to the thing, it looks equally bright.
What we perceive as brightness has to do with both a.) how much light gets to our eyes and b.) how much surface area we see. With Lambertian things, although the surface area your eye sees changes as you change your viewing angle, the amount of light you see changes too, in a proportional way.
The true definition has to do trigonometry and with the dividing an amount of light by the surface area it comes from. We won’t get into that here, though, because this description is for laymen, and you frankly probably don’t need to care that much. If you do care that much, look up Lambert’s cosine law for further reading.
A white sheet of matte (not shiny) paper is a good way of visualizing Lambertian scatter. For being an everyday object, it’s pretty close to a material with a truly Lambertian reflectance! Blocks of barium sulfate are frequently used for calibrating lab equipment because its reflectance is close to perfectly Lambertian, plus it reflects a super high percentage of light that hits it (usually at least 97-98% depending on wavelength, surface finish, etc.).
Truly Lambertian emitters are harder to find an everyday approximation of. One of these would give off light the same way a white piece of paper reflects light. If you look at the white part of some lit McDonald’s signs, this might be as close as we can get. The white letters would be about the same brightness whether you were in front of the sign looking at it, or standing to the right side of it, or the left side of it, or below it.