TINA is an acronym for "There Is No Alternative".
It is no alternative to stocks and being invested in the stock market. In a very low interest rate environment it is difficult to get any return in the market. Bonds and Currencies give very low returns.
Originally the term originated with the Victorian philosopher Herbert Spencer and became a slogan of British Prime Minister Margaret Thatcher in the 1980s. The phrase was used to signify Thatcher's claim that the market economy is the only system that works, and that debate about this is over. In economics, politics, and political economy, others have used it to mean that "there is no alternative" to neoliberalism—that free markets, free trade, and capitalist globalisation are the best or the only way for modern societies to develop.
In todays market the terms is used slightly different to explain the need to be over-exposed in the stock market because other interest rate linked assets offer low return.
The TINA-effect is the rise of the stock market because investors are shifting capital from bonds and into the stock market. And that the stocks rise only because investors have no alternative. In this scenario the stock markets get pumped up because of capital inflows instead of improving underlaying business in the companies of the stocks.