d1pole is a design for the centerpiece of a dipole antenna. It can be 3D-printed or laser-cut.
I am only an armchair antenna builder, but the following might help: A dipole antenna is usually a half-wave antenna, so the total length is half the wavelength of the corresponsing frequency. The wavelength at 144MHz is 300000000m/s / 144000000 Hz = 2m. Half that would be 1m, and because the dipole has two arms, each arm is again half that length, so we need 2 arms with 0.5m length each for 144MHz. We get the same by plugging the 144Mhz in the formula on the d1pole page (75/Mhz = length).
The coiled part on the bottom is a balun (well, technically it's not really a balun, but an air choke), which is needed because a dipole antenna is balanced and the feeding coax cable cable is unbalanced. This has to be compensated for, otherwise the feeding cable becomes part of the antenna, which is not desired.
So my attempt for a mini build howto: