It is the non-acceptance that Jews are (also) a people and that their national aspirations in the land of Israel (not necessarily all of it) are legitimate. Until the creation of Israel (1948), anti-Zionism was almost exclusively Jewish. There were three anti-Zionist positions (orthodox, socialist and liberal) that had their Zionist counterparts: the national religious (national recovery as the beginning of redemption), socialist Zionism (the contribution to world socialism would be done via a self-experience of cooperative construction) and the liberal and free-thinking Zionists, who understood that for Jews to genuinely assimilate to others, they should do it as a people and not by diluting their identity.
The first and bluntest opposition to Zionism came from the Arab and Islamic world since the 1920s and not from the world’s left-wing. During the 1950s and 60s, Israel was a symbol for the world’s socialists. The Arab Palestine leadership, under the disastrous Mufti Hajj Amin Al-Husseini and others, rejected the right of the Jews to self-determination.
The left had increased its criticism towards Israel because of the structures of the global alliances (with the U.S. supporting Israel and the left-wing movements of the world aligning with the Soviet Union and with Cuba in our continent). With the Berkeley Revolt (1964) and the French May (1968), the New Left began to attack Israel, accusing it of being a “colonialist manifestation”, despising its national identity, beyond whatever the Israeli government did or did not (at the time and until 1977 in hands of the Socialists).
At present, those who used to hate the Jews, find that demonizing the State of Israel and Zionism is more “politically correct”.