what is the debris?

it is extinct manmade objects in space, mostly in Earth orbit, that no longer serve a functional purpose Derelict spacecraft (nonfunctional spacecraft and abandoned launch vehicle stages), mission-related debris, and, especially in Earth orbit, fragmentation debris from the disintegration of derelict rocket bodies and spacecraft are examples. Other examples of space debris include pieces from their disintegration, erosion, and collisions, as well as paint flecks, solidified liquids discharged from spacecraft, and unburned particles from solid rocket engines. Space debris poses a threat to spacecraft. 

how debris affected NASA's spacewalk : 

NASA postponed a spacewalk set for today outside the International Space Station early this morning after receiving notification of a potential safety danger from nearby space debris. The source of the debris is unknown, but the delay comes around two weeks after Russia blew up one of its own satellites in orbit, resulting in thousands of deadly bits that endangered the space station.

NASA astronauts Kayla Barron and Thomas Marshburn were expected to wear spacesuits and depart the confines of the International Space Station (ISS) today morning at roughly 7:10 a.m. ET to replace an antenna on the station's exterior. It would have been Marshburn's seventh spacewalk and Barron's first.

However, before they could exit the airlock, NASA employees in mission control halted the spacewalk. 'It's just real life, this is how things work out sometimes, and I'm very grateful you gentlemen are watching out for our safety,' one of the astronauts onboard the ISS, Mark Vande Hei, told flight controllers in mission control, according to CBS News.

NASA stated in a brief blog post that it did not have enough time to 'fully analyze the risk' that the debris posed to the crew, so the spacewalk was postponed until further notice.

Space debris has disrupted preparations on the International Space Station for the second time in two weeks. On November 15th, mission controllers awoke the astronauts onboard the ISS exceptionally early and urged them to shelter in place after Russia performed an anti-satellite, or ASAT, test. Russia destroyed one of its own satellites with a kinetic missile, resulting in at least 1,700 pieces of trackable debris and hundreds of smaller fragments that cannot be traced. The debris cloud went close to the ISS on several occasions since the satellite was orbiting pretty close to it.

what are solutions to reduce debris :

There are four methods for removing debris from extensively frequented orbits:

(1) deorbiting (the deliberate, forced reentry of a space object into the Earth's atmosphere by applying a retarding force, typically via a propulsion system) at EOL;

(2) planetary lifetime minimization (accelerating the natural decay of spacecraft and other space objects to reduce the time that they remain in orbit) at EOL;

(3) moving objects into less populated 'disposal' orbits at the end of their functional lifetime;

(4) energetic cleanup from orbit.