As transportation gets faster and more efficient, the world effectively gets smaller. The physical size/area of the world doesn't change of course, but our perception of it gets smaller since we're able to travel greater distances in a shorter amount of time. Before the invention of the train, people relied on horses to get around and they didn't move very far on average. Afterward, trains allowed people to migrate further. Planes allowed for countries on different continents to connect more than ever before. These are examples where progress in innovation makes people closer together.
Because the world is getting metaphorically smaller, various cultures interact with one another to make a melting pot. Various ideas and concepts assimilate and mix with one another. Similarly, language is mixed as well which is a big part of culture. The same can be said about different foods. Since many people are very religious and highly value their customs, they may be resistant to change from outside forces. This is likely why indigenous cultures prefer to keep to themselves. Also, there have been many historical events in which external parties have been hostile to indigenous culture (eg: colonists calling tribes "savage"), which is a barrier to globalization.
Diffusion is the transmission of an idea, food, trend, etc from one culture to another. Think of it like a message transmitted. An example would be the Falafel which is a food that originated somewhere in the Middle East or Northern Africa (the true origin is unknown and still debated). One barrier to diffusion is the idea of taste/preference. If people don't like a certain item, then they'll be unwilling to spread said item onto other people in their culture. This inhibits growth of the concept. Another barrier to diffusion is immigration quotas. These are limits set by the government that prevent people from coming into a country. With more limits, this reduces diffusion further because there are less people to introduce a foreign concept from another culture.
