I'm an international relations masters student focusing on Asia. I'll give the basics: 1. Article 9 of Japan's constitution allows no use of offensive force to solve international disputes. Abe's reinterpretation allows for collective defence. 2. Japan maintains a relatively high technological military given it's partnership with the United States and it's global leadership in electronics. Big companies like Kawasaki or Mitsubishi actually create dual use technology for public and military use. Japan also manufactures some of it's own weapons like the F-15, Aegis ships, XAAM-4 air to air missile and TK-X tank. 3. Despite their high tech advantage, there are two major complications. First, the Japanese are seen to be vastly untrained and inexperienced according to US officers. Their training is very limited and equipment either in shortage or outdated. The second is it is purely defensive. Kenneth Waltz mentioned a purely defensive can't deter as they have no ability to inflict commensurate punishment. 4. The majority of Japanese citizens are anti militarism and enjoy the Yoshida Doctrine style of foreign policy Japan has maintained since the end of the second world war. This means no mostly military, the United States handle security issues and Japan focuses only on economics. 5. Rivalry. Despite relations normalizing between Japan and China in the early 1970s, they've essentially plummeted since. Senkaku/Diaoyu, gas fields, textbook issues, Yasukuni visits, and air identification zone overlapping are just few of the problems preventing any progress not to mention anti-each nation protests throughout recent years. North Korea is self explanatory.

The US desires a pivot to Asia to balance the area from a potential aggressive China and most importantly a rogue North Korea. Japan likes this as it helps protect them under the nuclear umbrella, China tolerates it as it prevents a re militarized Japan and several other nations like Vietnam and Philippines like it as it balances against China.

There is a ton more basics behind it but this should give you the idea.