Re. {slightly edited}
https://www.colorado.edu/physics/phys1230/phys1230_fa01/topic14.html
"Huygens’ principle (Christaan Huygens, 1629-1695, published about 1690) describes how a wavefront moves in space. According to this principle, we imagine that each point on the wavefront acts as a point source that emits spherical wavelets. These wavelets travel with the velocity of light in the medium. At any later time, the total wavefront is the envelope that encloses all of these wavelets. That is, the tangent line (or surface) that joins the front surface of each one of them."
The key words here are 'we imagine'. Huygens' Principle as used now is intended to be a model of wave propagation. It is not now used to describe the actual physics ( for that we need strings, springs, air, water, etc. ....). However Huygens originally did intend it to describe the physics--see Treatise on Light https://www.gutenberg.org/files/14725/14725-h/14725-h.htm
To answer your question:
A secondary wavelet means an imaginary point source at every point of the original wavefront emitting a spherical wave of the same form (or shape in time) as the original wave.