Short answer: he was sussing out support for India's independence from Britain, and possibly discussing transport to Southeast Asia to further his cause.

The long answer is he was a freedom fighter who was agitating for independence by any means necessary, including armed struggle, before the war. That placed him at odds with Gandhi, who insisted on nonviolence. The British placed him under house arrest, but he escaped and ended up in Moscow where he tried to gain support from the Soviets for India's independence.

The Soviets weren't interested so he went to Nazi Germany. There he found more support and the Nazis helped him form an Indian army from Indian POWs who had been fighting for the British.

By 1942 though, it was clear that Nazi Germany wasn't going to help drive the British out of India, especially when it was busy fighting on the Eastern front. Bose's only meeting with Hitler (in 1942) confirmed this, however Hitler did offer a submarine if Bose wished to go to Southeast Asia, where Axis power Japan had conquered the British colonies of Malaya and Singapore.

In 1943 Bose boarded a German submarine for Southeast Asia. The Japanese allowed him to recruit among the Indian POWs, and also to carry out fundraising among the Indian population of Singapore and Malaya.

Eventually, Bose's army, funded by donations and the Japanese, fought alongside Japanese forces in India. After the Japanese were defeated there it fought with them in Burma (once a British colony, then conquered by the Japanese) before being defeated there, too.

Right after Japan's surrender Bose left Singapore and died in a plane crash. There are conflicting accounts of where the plane was headed. There are also those who say he didn't die at all.

Today Bose is less well known internationally than Gandhi, but still well known in India. His legacy is complicated, but personal views I've come across acknowledge him as a freedom fighter. Some admire him for his strength and his founding of an army. One told me Gandhi was "weak" and Bose was "a real fighter". Others feel he was tainted by his association with the Japanese Empire and its cruel treatment of its conquered subjects. All those I spoke to admired him for creating an inclusive Indian army that included a female unit.