I heard that Turkey has launched strikes against Syrian Kurds. Why is Turkey is doing that?
It's part of a longer conflict between the Turks and the Kurds. From Wikipedia's article on Kurdish–Turkish conflict (1978–present):
The Kurdish–Turkish conflict is an armed conflict between the Republic of Turkey and various Kurdish insurgent groups, which have demanded separation from Turkey to create an independent Kurdistan, or to have autonomy and greater political and cultural rights for Kurds inside the Republic of Turkey. The main rebel group is the Kurdistan Workers' Party or PKK (Kurdish: Partiya Karkerên Kurdistan). Although insurgents have carried out attacks in many regions of Turkey, the insurgency is mainly in southeastern Turkey. The PKK's presence in Iraq's Kurdistan Region, from which it has also launched attacks, has resulted in the Turkish military carrying out frequent ground incursions and air and artillery strikes in the region. The conflict has cost the economy of Turkey an estimated $300 to 450 billion, mostly military costs. It has also affected tourism in Turkey
Why is Turkey considered to be helping ISIS?
Because the Kurds keep many ISIS fighters as prisoners and an armed conflict may provide opportunities for prisoners to be freed or escape. From the Reuters article:
The Kurdish-led authority in northern Syria said a prison struck by Turkish shelling holds “the most dangerous criminals from more than 60 nationalities” and Turkey’s attacks on its prisons risked “a catastrophe”.
The Kurdish-led Syrian Democratic Forces (SDF) holds thousands of Islamic State fighters and tens of thousands of their relatives in detention.
But world powers fear the operation could intensify Syria’s eight-year-old conflict, and runs the risk of Islamic State prisoners escaping from camps amid the chaos.