Senbero is a slang term for an izakaya (Japanese Style Pub) where you can get drunk for 1,000 yen.
In general, senbero is a reasonable izakaya or standing bar where you can have a couple of drinks and snacks for about 1,000 yen. Because of the low price, many people tend to go to several izakayas to have a drink.
In Tokyo, Tateishi in Katsushika City, Akabane in Kita City, and Ueno in Taito City are said to be mecca for senbero, and many izakayas that fall under the senbero category are gathered there.
In addition, you can enjoy senbero in various areas of Tokyo, such as Shinbashi, which is an office district and mecca for businessmen with many izakaya in the vicinity, and Shinjuku and Shibuya, which are busy entertainment district.