What is the meaning of "Shol Tasholu"?
Rashi #1: Forget. Pretend to forget (Malbim - so there will be Shichechah for her to take). The Targum of Shegagah is always Shalusa. The same applies to "Al ha'Shal" (Shmuel II, 6:7; Hashem struck Uza for forgetting).
Rashi #2: It is an expression of dropping (so there will be more Leket for her), like "Ki Yishal Zeisecha" (Devarim 28:40).
What are "Tzevasim"?
Rashi: They are small piles. We find like this in the Mishnah - if one found Tzevasim or bundles (Menachos 10:9).
Why did he say "va'Azavtem"?
Malbim: Since you forget them intentionally, they are not really Shichechah; she will not know that Terumos and Ma'aseros must be separated! Therefore, make them Hefker 1 ; Hefker is exempt from Terumos and Ma'aseros.
Since Bo'az owns the field, he himself can make them Hefker! He told the workers to do so, for he did not want to rely on Bereirah (to make Hefker now what the workers will leave later - Bava Kama 69a). (PF)
He already warned them not to shame her (15). Why did he add "v'Lo Sig'aru Vah"?
