A spin-echo sequence, one repetition: an RF pulse tips magnetization, gradients turn on and off in a precise order, and a signal is read out — then the whole thing repeats every TR. Drag TE to see the 180° pulse and the echo move together (the echo always forms at TE); drag TR to see how much longer the scanner waits before repeating.
Reading the diagram: the 90° pulse (with the slice-select gradient Gz on) tips magnetization into the transverse plane. A brief Gy phase-encode blip follows — a different amplitude every repetition, which is how the phase-encode direction gets filled in, one k-space line per TR. A negative Gx lobe deliberately dephases the spins; the 180° refocusing pulse at TE/2 flips them so they rephase into an echo exactly at TE, which is when the positive Gx readout lobe is on and the signal is actually sampled. Everything after that is dead time until the next TR, when the whole block repeats with a new phase-encode amplitude.